1. Field of the Invention
This invention relates to a screw compressor.
2. Description of the Related Art
The pressure of an intake channel and the pressure of a discharge channel in a screw compressor are determined by both an air charging unit (an atmospheric pressure in a case of sucking atmospheric air) and demand equipment. On the other hand, the pressure of gas obtained immediately before the gas is discharged from a rotor chamber to the discharge channel in the screw compressor is determined by the pressure of the intake channel and a mechanical compression ratio (a volume ratio) of the screw compressor. When the pressure of gas obtained immediately before the gas is discharged from the rotor chamber is higher than the pressure of the discharge chamber, the gas will be expanded at the moment when the gas is delivered into the discharge chamber, resulting in a drop of the pressure. Therefore, all power used for compressing the gas by an amount corresponding to a difference between the pressures will be wasted.
Some of the screw compressors comprise a slide valve for changing the degree of opening of a discharge port and have a capability of adjusting the mechanical compression ratio as described in Japanese Patent H09-317676-A, for example. However, the slide valve is complex in structure and significantly increases costs. Moreover, the slide valve has a drawback of requiring complex control.